Image analysis – Pattern recognition – Feature extraction
Reexamination Certificate
1998-01-08
2001-05-22
Lee, Thomas D. (Department: 2722)
Image analysis
Pattern recognition
Feature extraction
C382S266000
Reexamination Certificate
active
06236754
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to methods for trapping images wherein adjacent pixels within an image area use differing colors which sometimes result in an annoying white border between color transitions and, more particularly, to a method and system of trapping images rendered in a continuous tone raster wherein component-wise maximum positive colorant differences are determined from neighboring pixels for each pixel a fraction of said difference is then added to the pixel's colorant values resulting in values somewhere between the maximum neighboring and original values for each color component resulting in an image that is less susceptible to gaps because of misregistration at color transitions
BACKGROUND OF THE INVENTION
Color printing is typically carried out by printing three or four color separations, (eg. cyan, magenta, yellow and black). Intermediate colors are made by printing mixtures of various amounts of these primary colors. It is possible, however, to specify pairs of colors that have little or no common components. For example, pure cyan and pure magenta, or an orange made from yellow and some magenta and a dark cyan made from cyan and black If adjacent areas of an image use two such colors, then a potential problem arises. The problem occurs if the registration between color components is not perfect. If the color separations are not perfectly aligned, then a gap can occur at the boundary between the two colors and the white of the paper can show through. This is considered objectionable and a technique known as trapping is used to reduce it. Trapping is usually thought of as moving the boundary for one of the colored areas so that it overlaps the neighboring area. If one is dealing with graphical objects, this amounts to spreading or bloating an object, or in choking or shriking its background
The layout of a page or graphic image depends upon combining “structured graphics” according to a pre-established graphic design The structured graphics are contiguous regions of color, usually represented in a plurality of separation images, which represent a succession of graphic objects imaged on the printing medium (e.g. the “paper”). The objects so imaged are shapes which can be isolated from each other, can abut one another at one or more points, can partially overlap one another, or can completely overlap one another. The resulting printed page or graphic image is therefore made up of a patchwork of shapes representing the graphic objects, some of which are “clipped” by objects imaged later in the succession.
The result of abutting or overlapping shapes is a boundary between adjacent regions of color which, under ideal printing conditions should have zero width. That is, the one color should stop exactly where the other begins, with no new colors introduced along the boundary by the printing process itself. The “colors” which fill the shapes can be solid colors, tints, degrades, contone images, or “no fill” (i.e., the paper with no ink applied). In general, the “colors” represented in these adjacent regions are printed using more than one colorant. In practice therefore, the realization of a zero width boundary between regions of different color is impossible as a result of small but visible misregistration problems from one printed separation to another. The error is manifested as a “light leak” or as a visible boundary region of an undesired color.
The problem of misregistration is a mechanical problem, almost always existing in printing systems. The problem arises because color separations are not laid exactly where intended, due to inherent imperfections in any separation registration process. It is somewhat correctable by mechanical registration methods; however it is rarely completely correctable. In expensive, high end printing processes, customers have high expectations that misregistration artifacts will not be visible. In inexpensive, low end printers, mechanical registration techniques are so expensive as to make correction or trapping essential.
As will become apparent, different printing technologies have distinct misregistration artifacts. Offset printing tends to have uniform misregistration in all directions. However, xerographic printing tends to have misregistration primarily in a single direction.
Methods for correcting for this misregistration are known in the prior art. The general approach is to expand one of the regions which abut so as to fill the gap or misregistration border region with a color determined so as to minimize the visual effect when printed Borders which are expanded from a region of one color to another in this manner are said to be “spread”. A border which has been so expanded is referred to as a “trap”, and the zone within which color is added is called the “trap zone”.
Commonly used methods for automatic trapping of digital images fall into the categories of vector-based and raster-based methods. Vector-based methods rely on images that have been converted from a page-description language form, describing objects as characters, polygonal shapes, etc. into an internal data structure containing not only object information, but also a list of all the edges between regions of different color. Raster-based methods rely on images that have been first scanned or converted for page-description based form and are stored internally as a sequence of (high resolution) scan lines each containing individual scan elements or pixels. These methods process each raster line in sequence and compare one or more adjacent pixels to determine color boundaries. After some initial processing to find edges, both vector-based and raster-based methods apply rules for determining whether or not to create a trap at such boundaries, and finally apply a second set of rules to determine the nature of the trap if one is to be created.
Thus, it can be seen that most trapping processes takes the following format which shall be referenced throughout this discussion
A. Find edges in the image, no matter how described;
B. For each pair of colors on each side of the found edge, determine:
1) Whether trapping should be used, and
2) choose a method of trapping.
Modify the image accordingly.
The present invention differs from this prior art in that there is no explicit finding of edges (no step A). It also differs from step B in that it does not try to decide whether or not trapping should be used; instead, it always applies its modifications to every pixel. Furthermore, there is no selection of a trapping method; its single modification scheme is universally applied. In the prior art, the method of Taniguchi, described in US-A 4,931,861, uses two rasterized images representing abutting or overlapping objects within an image field to define a third binary image representing the map of the pixels which make up the borders between the first and second images. These three images are superimposed, pixel by pixel, to create a fourth and final binary image.
The method of Darby et at., described in US-A 4,725,966, again defined on a pixel basis, uses a mask which is moved, one resolution element at a time, to evaluate the presence or absence of (pixel) colors upon which a positive or negative spread decision is based.
The method of Yosefi, described in US-A 5,113,249 uses a set of automated rules as the basis for deciding, for each pair of abutting or overlapping shapes whether or not to create a trap (an overlap region referred to as a “frame”), and, if so, the nature of the trap to create. The preferred embodiment described by Yosefi makes use of scanned data, and processes each line of pixels in order, comparing for each pixel three pixels from the previous scan line and two pixels from the same line to determine if a color change has occurred The decisions regarding whether or not to create a trap, and the nature of such a trap if created are imbedded within the processing sequence, making use of criteria established prior to the onset of processing. Yosefi describes rules to follow after finding an edge and knowing the two colors. Th
Brinich Stephen
Lee Thomas D.
Xerox Corporation
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